The determination of the strain and stress distribution at the surface of a mechanical or civil engineering structure under static or dynamic conditions is important for the proper design and safe operation of the structure. At present, two types of strain measuring device are used. Firstly, strain gauges, or tensometers, measure surface strain through the deformation of a gauge or sensor. Secondly, manually operated photostress devices, also known as birefringent coating polariscopes, utilize a thin layer of photoelectrically sensitive material glued to the surface being measured.
The photosensitive material distorts with the distortion or strain of the surface. This changes the light transmitting properties of the photosensitive material and when viewed under incident monochromatic light transmitted once or twice through the photosensitive material, the material exhibits characteristic interference patterns. The interference pattern is composed of light and dark fringes. The distribution of the light and dark fringes in the interference pattern gives, after appropriate analysis, information about the strain and stress distribution on the surface of the structure. The stress distribution on the surface forms a generalized vector, or tensor.
Present photostress devices enable the viewing and sometimes also the measurement of the intensity of the interference pattern, but only give information about one dimension of the three dimensional stress tensor.
Both strain measuring methods are time consuming and are not easy to use. The measurements must be performed by highly qualified personnel, especially in the case of photostress analysis.
The first method requires the application of many strain gauges to estimate accurately the state of strain on the measured surface. This is onerous, time consuming and potentially inaccurate due to the sensitivity of the gauges to temperature changes.
The second method, using the birefringent coating polariscope, can be performed only under laboratory conditions and has the following additional disadvantages.
(1) It does not provide strain and stress values immediately.
(2) The measurements give only information about the state of strain at one point of the surface. At least two additional measurements are necessary to estimate the strain state of the measured object.
(3) The method requires further application of another procedure or numerical calculation to obtain the stress distribution in the structure.
(4) The method does not readily give measurements over time.